Meta Box

Thanks guys! I got it sorted in the end with the following:

function XXXX_readable_month_year_event($post_id) {
    if (get_post_type($post_id) == 'event') {
        $event_date = get_post_meta($post_id, 'event_date', true);
        $readable_date = date('F, Y', $event_date);
        update_post_meta($post_id, 'event_readable_my', $readable_date);
    }
}
add_action('save_post', 'XXXX_readable_month_year_event');

What that does in order is:

1) Checks the post type is "event"
2) Gets the field "event_date"
3) Turns "event_date" into human readable format Month, Year
4) Adds the readable format as text to the Metabox cusom field "event_readable_my"
5) Runs on save

I think I explained badly what I needed to happen originally, but I got there in the end lol